Background:Malignant squamous cell differentiation of an epidermoid cyst can carry a significantly poor prognosis and very little is known about this entity.

Case Description:We present the case of a 35-year-old lady, with primary malignant squamous cell carcinoma (SCC) arising from a previously partially resected cerebellopontine angle epidermoid cyst almost 5 years after initial resection. We also review the relevant literature.

Conclusion:The imaging findings, histopathology, and management of a malignant SCC arising from a benign epidermoid cyst are discussed with recommendation for increased surveillance and follow-up, even for classically accepted benign lesions.

Epidermoid cysts are benign, slow growing, extra-axial tumors that account for ~1% of all intracranial tumors. Embryologically, they are derived from ectodermal inclusions during neural tube closure.[6] Intracranial squamous cell carcinoma (SCC) is commonly seen as a metastatic deposit, however, there are case reports of malignant transformation of epidermoid into SCC. We present the case of a 35-year-old female with primary malignant SCC arising in a partially resected cerebellopontine angle (CP angle) epidermoid.

The patient originally presented in 2010 with headaches. Neurological exam was unremarkable. Magnetic resonance imaging (MRI) demonstrated a CP angle lesion that was lobulated and predominantly of low signal intensity in the prepontine and right ambient cistern, with a few small foci of increased signal intensity on diffusion imaging, which was consistent with an epidermoid along with a small left-sided acoustic neuroma. At that time, a right retrosigmoid craniotomy was performed with the intention of decompressing the brainstem and cranial nerves. The right CP angle lesion was partially resected with a small residual left coating the anterior brainstem in the prepontine cistern. Histology demonstrated an epidermoid tumor with typical histological features of lamellar keratin and benign squamous epithelium. The small residual was monitored with serial MRI scans. Approximately 5 years later, routine follow-up MRI demonstrated marked growth of the residual lesion with development of a multilobulated enhancing cystic component. The patient complained of a 6-month history of right-sided facial numbness. After discussion with the patient, the decision was made to reoperate and resect this lesion. A repeat right retrosigmoid craniotomy was performed. Intraoperatively, the lesion was cystic with an anteromedial solid component adhering to the lateral aspect of the brainstem. There appeared to be a dural attachment which was resected. The patient awoke well initially, however, subsequently developed a House–Brackmann grade 5 facial palsy on day 2 [Figure 1].

Malignant transformation of an epidermoid tumor into SCC is a rare but described phenomenon.[145891011] By far the most common cause of intracranial SCC is metastasis from elsewhere. True primary SCC is thought to arise from malignant transformation of a dermoid or epidermoid within the central nervous system.[37] Garcia first described this in 1981 and proposed a set of criteria for the diagnosis of primary squamous cell carcinoma; Tumor restricted to the intracranial compartment, no invasion or extension beyond the dura or cranial bones, no communication with the middle ear, air sinuses or sella, no evidence of nasopharyngeal tumor.[3] Hamlat et al. added to this in 2005, proposing that there also be benign squamous epithelium within the main tumor mass and no evidence of a primary tumor elsewhere.[4] Prognosis has been considered poor across the literature with a median survival of 9 months, although this has been increased to 26 months in some cases with radiotherapy.[2] Any rapid growth of a lesion in the site of a previously resected epidermoid or dermoid or change in the MRI characteristics of any residual should be considered suspicious and prompt either operative intervention to gain tissue for analysis or a more frequent surveillance schedule.